In general, to prevent vaporized fuel in an automobile fuel tank from leaking into the atmosphere, a canister filled with active carbon that adsorbs vaporized fuel is connected to the fuel tank, and vaporized fuel is adsorbed by this active carbon when the vehicle is at rest. The vaporized fuel adsorbed by the canister is discharged from the active carbon by negative intake pressure when the engine is running, and the air led into the canister, and is then supplied to the air intake pipe of the engine.
The canister and the intake pipe downstream of the engine throttle are connected by a purge passage. A purge cut valve is provided in the purge passage.
Even in this mechanism, however, If a leak occurs in the flowpath from the fuel tank to the intake pipe due to changes as a result of aging, etc., or the seals in the joins of the pipes constituting the flowpath are defective, vaporized fuel is released into the atmosphere.
The Environmental Protection Agency (EPA) and the California Air Resources Board (CARB) require that checks be performed to determined whether or not the leak amount is below a tolerance value, and that measures are taken to prevent leakage into the atmosphere if it is not. These bodies also recommend apparatuses and methods for diagnosing leaks.
In one such apparatus, an air supply valve that opens and closes the air intake passage of the canister, and a sensor that detects the pressure in the flowpath leading from the fuel tank to the purge cut valve, are provided. First, the air supply valve is closed and the purge cut valve is fully opened so that the negative pressure in the air Intake pipe downstream of the throttle is led to this flowpath, then the purge cut valve is shut so that the flowpath is sealed. If there is a leaky part in the flowpath, the pressure in sealed flowpath suddenly returns to atmospheric, whereas if there is no leak, the pressure gradually rises due to vaporized fuel generated in the fuel tank. Hence, if this pressure is monitored, it is possible to diagnose the existence or absence of a leak.
However, this test applies to the whole flowpath from the fuel tank to the purge cut valve, and when a leak is detected, it is not possible to determine what specific part of the flowpath has a leak.
Moreover, as purge control must be interrupted during the leak diagnosis, it is desirable that the leak test is completed in a short time. According to the above method, however, the pressure must be compared when the pressure in the flowpath has risen to a certain level due to generation of vaporized fuel in the tank, so the test takes some time.
Further, when the engine is in the idle state, the purge cut valve is generally closed and the vaporized fuel mechanism is set so that purge is not performed in order to maintain driving performance. The above apparatus therefore does not perform a leak test in the idle state. If the engine enters the idle state during a leak test, the whole test is stopped, and the test is repeated from the beginning when the running conditions are once again suitable for test. Consequently, during running conditions when the engine often enters an idle state, a test for the presence of a leak cannot be performed.
In the case of the above test, determination of the presence or absence of a leak may be made for example by calculating the ratio of the time taken for the flowpath to reach a predetermined negative pressure due to introduction of intake negative pressure into the flowpath, to the time taken for the flowpath pressure to return to a predetermined value from when the purge cut valve is shut, and comparing the result with a preset reference value.
In this case, if the accelerator is depressed while negative pressure is being introduced into the flowpath, the intake negative pressure becomes weaker so that the time required for the flowpath to reach the predetermined negative pressure increases. There is then a risk that the extent of a leak may be estimated to be greater than it really is.